What does a one-in-ten-million chance of apocalypse look like? Well, it used to look like this:

That is asteroid 2005 YU55, a near-Earth object (or NEO) that also happens to be a PHA, or potentially hazardous asteroid. It has an orbit which intersects the Earth, which means that someday it could possibly hit us.

Now before you panic — and I’ll make this clear: DON’T PANIC — that doesn’t mean you’ll wake up tomorrow to see flaming death streaking across the sky. Think of it this way: when you walk to the local convenience store to get a squishy, you have to cross the street. The path you take intersects the street, but as long as you don’t try to occupy the same spot as a moving car, you won’t get hit. Same with PHAs: their orbits cross the Earth’s orbit, but space is big. As long as the Earth and the asteroid aren’t at the same place at the same time, we’re OK.
Since we don’t know the orbits of these objects perfectly, we assign a probability they will hit us over some period of time. Up until recently, YU55’s chance of hitting us over the next century was calculated to be about 1 in 10,000,000, which is reasonably close enough to 0 for me. However, it’s always good to get better data. In this case, very good: new observations have eliminated the chance that YU55 will ruin our day for at least a century to come.

YU55 was observed with the monster 300 meter (1000 foot) Arecibo radio telescope in Puerto Rico. Arecibo can send little radar pings into space, aimed at an asteroid. The pings reflect off the rock, come back to Earth, and the timing of each one can be logged. This tells us how far away the asteroid is, how big it is, and even (by carefully measuring the different arrival times of the pings back on Earth) the shape of the asteroid.

If this sounds familiar, that’s because this is how dolphins and bats sense their environment. They use sound, not light, but the principle is the same. So what did Arecibo tell us when it dolphinated YU55?

The good news is that the orbit of the asteroid was nailed down better, and that 1 in 10,000,000 chance of an impact in the next century dropped to 0. Nada. Nil. And astronomers are so confident of that they removed YU55 from their Risk Page.

So we’re safe from YU55 ruining our day for quite some time at least.

And that’s good, because, as it turns out, YU55 is bigger than expected: about 400 meters (a quarter mile) across, twice as large as previous estimates showed! Something that big hitting us at orbital speeds would explode with the force of a lot of nuclear weapons — a few thousand megatons, or a hundred times the yield of the largest bomb ever detonated.

So yeah, yay! It won’t hit us, and that’s by any definition good.

But the middlin’ bad news is that this also means is that it’s tough to get good size estimates for asteroids without this technique. Usually, the size of a rock is determined by measuring how bright it is. A bigger asteroid reflects more light, and by measuring how well it reflects sunlight we can estimate the size. But that doesn’t always work so well, as YU55 is telling us. Clearly, we need to use multiple methods to get the sizes of these guys.

Arecibo’s funding is constantly under attack, yet it’s the best machine we have to get the sizes of and, more importantly, accurate orbits for these potentially life-threatening objects. YU55 is off the list now, but there’s a long line of rocks ready and waiting to take its place there.

So this thing goes around the sun in a high-eccentricity orbit that occasionally crosses paths with Earth? That’s the first reason off the top of my head that it could be a threat both soon and a century later. Or is our time-resolution on NEOs even more atrocious than the size estimates? Where is this thing like right now?

Routine planetary violence, as Carl Sagan put it…. It’s not IF it happens, it’s WHEN. Unlike the other species on the planet we can do something about this threat, and with present technology. Another major impact is due in the next couple of million years and it would be ironic indeed if we had the tools at our disposal, but decided to follow T Rex’s example. Anybody serious in supporting efforts to do something about this should join The Planetary Society: http://www.planetary.org/programs/projects/mirrorbees/

It may not be completely accurate to refer to radio frequency as “light” but of course it’s all the same phenomena (electromagnetic radiation) and moves at the same speed, so it’s pretty common to use light informally for any EM frequency. Given Phi’s other creative use of English, that part doesn’t bother me a bit.

Close, but not quite. We don’t “assign” a probability – we calculate one. Imagine you’re measuring the width of your lot. You have six measurements that average to 100 feet, BUT, there is a tiny chance that ALL your measurements are too small. There’s a 50% chance that any given measurement will be below the real value, so the chance that all six are low is 1/2 x 1/2 six times, or 1/64. There are six ways to have only a single measurement too small, so the odds that 1 of the 6 measurements is too small is 6/64. The odds that two are two small is 15/64, and an even split (three too small, three too large) is 20/64. You may recognize the binomial series here.

So odds of an impact means, given the measurements we have, what is the chance that we’re off sufficiently that the object could hit us? It will either hit or not and that’s determined by the laws of physics. What we don’t know is how precisely our measurements match reality. And that is true no matter how accurate our measurements are – there is always a limit to accuracy. The odds, before the new data came in, were about one in 10 million that we were that far off. It’s like drawing a straight line with a yardstick. Use two points a foot apart and your line will have some small error. Use points three feet apart and the error is a lot less. We have more measurements over time for the asteroid, and a more accurate orbit.

MoonShark, courtesy of JPL’s Horizons site, at 0 GMT on 30 April the asteroid is at RA 02 43 49.57 Dec. +15 52 34.2. We had closest approach April 19. It will be 0.08170536516031 AU away increasing about .008 per day. So, yeah, we know where it is pretty precisely. After all, how did Arecibo know where to point their antenna? The question is, will some tiny error in the last few decimal places compound over time to something important a few centuries from now?

If your picture is out of focus and pixellated enough you can claim it’s anything. This is just another UFO hoax, right? 😛

Now as for the comments about Arecibo pointing it’s antenna – don’t imagine that the enormous dish moves – it doesn’t (well, not relative to the earth anyway). The small amount of pointing that can be done is achieved by moving the receivers (that enormous housing and its appendages suspended above the dish). I think it’s quite remarkable that it can actually ‘point’ as much as 22 degrees from the zenith:

If this sounds familiar, that’s because this is how dolphins and bats sense their environment. They use sound, not light, but the principle is the same. […].

In the context of that paragraph, Dr. Plait was clearly referring to dolphins and bats using “sound, not light”, when he used the pronoun “they” — which is used to represent the persons or things last mentioned or implied — to mean that dolphins and bats ‘see’ with sound waves rather than light waves. I do not think that he meant to imply that the Arecibo radio telescope ‘sees’ with “light”.

@Eric Goetz (#27):
Well, Arecibo has taken a radio image of Saturn using the radio energy they supplied. (Admittedly, Saturn is a considerably larger target than an asteroid.) But imagine the spotlight you’d need to light up a whole planet that was otherwise dark so you can photograph it! When they are sending radio out, they have to clear the area around the dish to keep people from getting blasted with leakage from the dish, if that tells you anything about the power levels (i.e. radio waves are really wimpy in terms of the energy they carry, so you have to make a lot of them to do any hurt to people/things).

About a year ago, someone in Congress was railing about, “wasteful spending on volcano research.” That was shortly before several rather severe earthquakes – and there IS some connection between volcanos and earthquakes, not to mention our friends in Iceland. Let’s hope they’re not equally prophetic about “wasting money on Arecibo.”

MadScientist is right in one respect. The Arecibo dish is static. But the receivers and transmitters can be tightly aimed. That’s what is on that big triangular platform hanging over the dish. They don’t image half the sky and then pick quasars a few seconds of arc in diameter, or the planet Saturn, out of the image. They focus on the object they want to observe. They didn’t blast radar omnidirectionally to get an echo off an asteroid 400 meters in diameter.

There is one slight difference between this NEO PHA crossing Earth’s orbit and my crossing the street for a squishy, I can stop to wait for the car to pass. Neither the Earth nor the NEO PHA can do that if they are on a collision course.

Is YU 55 its name or just its catalogue designation number – does it have a “proper” name or will it get one later?

While on names : Is there such a category as ex-PHA? Has this happened to many other asteroids?

Plus is it Near Earth Object or Near Earth Asteroid, pretty sure I’ve seen NEA as used as the acronymn before. Do both terms get used interchangeably still or has NEO replaced NEA? I know there are also comets that approach and cross Earth’s orbit too (& some overlap between objects that are half-comet, half asteroid) but I don’t think I’ve ever heard of them being described as either NEO’s or Near Earth Comets.

Does anyone at all use the IAU’s silly Small Solar System Bodies (or whatever it was) term?

It [2005 YU55] has an orbit which intersects the Earth, which means that someday it could possibly hit us.

Or the Moon or Mars or Venus? How far does YU 55’s orbit extend and is it only our globe in the firing line or are some other planets possible targets too?

That image is exactly what death from the skies will look like. A few months before some dark giant rock no one noticed before slams into us, we’ll see a vague, creepy image just like that one on our TV screens — threatening, ominous, almost shapeless.

And then CNN will make a bright shiny graphic of an asteroid slamming into the Earth, which they’ll reply over and over and over.

@Steve Dutch (24): Hey thanks! I didn’t mean to suggest astronomers were uncertain of the asteroid’s location — “where is it?” was supposed to be a separate question, just of out curiosity.

Anyway, if all those decimals really are sig. figs., that’s a fair bit better than I expected we could do. Impressive stuff, but of course it makes me wonder what we could do with even bigger radio equipment

@Messier In the designation 2005 YU55:
– 2005 represents the year in which the object was discovered
– Y represents the half-month, in this case Dec. 16-31 (I and Z are omitted)
– U is a second (counting) letter (I is omitted here as well)
– 55 is the number of times that the second letter has cycled through the alphabet

So the first objects discovered between Dec. 16 and 31 in 2005 would be 2005 YA, the second 2005 YB … the 25th 2005 YZ, the 26th 2005 YA1 etc. You can read more about that here.

2005 YU55 is just a provisional designation. Once the orbit of the object is determined to sufficient accuracy, it will receive a permanent designation, a number. After an object has been numbered, the discoverer has the ability to suggest a name for it.

An orbit can be improved through further observations so that objects once fulfilling the requirements to be classified as PHAs are moved to the “ex-PHA” list – which is available here.

NEAs are really a subset of NEOs, which do include Near-Earth Comets and man-made spacecrafts as well (for example). Thus NEO is a more generic term, I usually don’t see the term NEA used unless you’re talking about a specific object which you know displays no cometary activity (but please don’t quote me on that :D)

Currently, the Minimum Orbit Intersection Distance (MOID) calculated for 2005 YU55 is 0.00106 AU, i.e. about half the distance to the Moon. Actually, next year, on November 9, it will pass by at about 85% of the distance to the Moon. But, strictly speaking, the orbits don’t INTERSECT.

… and yes, the object also crosses the orbits of Mars (last close encounter on July 14, 2002) and Venus (May 6, 1993)

Kind of disappointing that Aricebo’s funding is forever being questioned. Maybe they need to create a separate district for it, represented by a powerful senator and have several of his friends and family on staff. Then not only would its funding be secure, but it would probably have its own navy too.

I’m puzzled by the illumination in that image; the energy is being delivered by a radar beam directed toward the asteroid from Arecibo in Puerto Rico. How can that produce this crescent we see? Even if another radio telescope located thousands of miles away were acting as the receiver, the separation between the two compared with the distance to the asteroid couldn’t produce an effect like this…could it? How about it? How does this work?